The Statistics::Contingency class helps you calculate several useful
statistical measures based on 2x2 "contingency tables". These can be
used for measures to help judge the results of automatic text
categorization experiments, but they are useful in other situations
as well.

The general usage flow is to tally a whole bunch of results in the
Statistics::Contingency object, then query that object to obtain the
measures you are interested in. When all results have been collected,
you can get a report on accuracy, precision, recall, F1, and so on, with
both macro-averaging and micro-averaging over categories.
